1. Field of the Invention
This invention relates to means for focusing a beam of electrons which is generated by the cathode of a cathode ray tube (crt) and controlled by electrodes such as a grid and one or more anodes to form an image ("the spot") on a screen.
2. Discussion of Prior Art
A conventional means of achieving this is to provide an electrode whose potential, usually at a value of several kilovolts, is to some degree variable to accommodate variations, within manufacturing tolerances, in tube dimensions and positions of the components within the tube and, if need be, drift in power supply outputs which affect the potential applied to other electrodes.
While it is a relatively straightforward matter to provide means for such occasional variations in focus electrode potential, problems are encountered in providing means for a rapid and continuous variation in the high value potential, typically several kilovolts, normally applied to the focus electrode.
Such a variation would be necessary to provide a "dynamic focus" facility, i.e. to maintain a sharp focus when the spot is not only at the centre of the screen, but also at the periphery of the screen at the extremes of its raster scan. This facility is required especially in view of the trend towards flatter and squarer tubes, which although presenting a less distorted image to the viewer present rather greater problems in maintaining spot focus to the corners of the screen, and also in the use of high brightness tubes, such as those required for aircraft cockpit displays and the like, the electron beams in which have a reduced depth of focus.